Six legs good

They developed architecture and built farms millions of years before we did. They work together so seamlessly that colonies are known as 'superorganisms'. And they could hold the secret to working out how our brains evolved. Alok Jha investigates the extraordinary world of ants

Without ants, the world would be a mess. Soil would be unable to sustain much life. Dead leaves, insects and small animals would litter the Earth's surface. Invertebrate pests would bloom, killing many of the food plants we need to survive. Thousands of species of flowering plants would disappear into extinction, robbed of a vehicle for their pollen.

Those mobile dots that can suddenly appear in sugar bowls, crawl in neat lines over shoes or ruin an otherwise perfect picnic are a silent, if annoying, pointer to one of the most successful forms of life on Earth. Here is a set of 14,000 species (with probably an equal number yet to be found) whose combined weight is equal to that of the world's entire population of humans. They have perfected life that is more collegiate than anything achieved by humans, and had developed architecture and built farms millions of years before our primate ancestors had even considered walking on two legs.

Ants have fascinated philosophers, writers and naturalists for thousands of years. But the last decade has seen a flurry of detail emerge about the natural history of the insects. Scientists have filled in huge amounts of detail on how the insects survive, communicate and, most importantly, how the thousands (sometimes millions) of individuals make collective decisions without any central leadership. That study has put ants at the centre of what many leading biologists say is a new phase of biology - understanding how groups of individuals can behave as a single "superorganism".

Edward O Wilson, sociobiologist at Harvard University and the world's leading authority on social insects, has studied ants for more than 50 years and says he finds it difficult to overestimate the importance of them to life on Earth. "They are the main turners of the soil - more important than earthworms - the main predators of small insects and the principal scavengers and garbage collectors," he says. "They capture insects and help keep things in balance and they are also the principal movers of small animals such as fallen sparrows and small rodents."

Since the first ants emerged more than 150m years ago, the insects have made it to every continent except Antarctica, filling every ecological niche as hunters, scavengers or farmers, and evolving into thousands of shapes and sizes. Leptanilline ants are less than 1mm in length and look like a dusting of pepper; the bulldog ants of Australia can grow to 5cm and each one packs a lethal sting for its victims.

To a greater or lesser extent, ants eat the same resources as other solitary insects but they have been far more successful. Why? "That's easy," says Wilson. "They live in groups."

Ant colonies range from a dozen individuals to millions of insects, mostly consisting of sterile females in specific jobs as workers, soldiers or caretakers, with one or sometimes a few reproductive females presiding over the entire brood. Any males are usually there temporarily, relatively useless drones kept long enough to inseminate the queen, then driven out of the nest or killed quickly afterwards. A queen can store the sperm from a male for a decade or more, using it over time to fertilise millions of eggs.

This system has worked well for them. "They really did, through the powers of evolution, discover the major principles in industrial revolution - but their revolution came tens of millions of years before ours," says Nigel Franks, a zoologist at the University of Bristol who specialises in studying social insects.

One of the most iconic industrial species is the Atta, commonly known as the leafcutter ant, found in the Americas. A typical colony contains up to 8m individuals, with the biggest around 200 times heavier than the smallest and each caste specialised for a specific task. The biggest ants cut the leaves from a tree or shrub with powerful jaws that can vibrate thousands of times a second. Another caste takes the fragments back to the nest and a third cuts up the leaf still further. The smallest ants then use the bits of leaves to make compost for the fungus that the colony farms for food. These ants also weed the fungus gardens and use antibiotic-producing bacteria to ensure their crop remains free from disease.

Their work rate is astonishing. A nest of Atta leafcutter ants can defoliate an entire citrus tree in a day and, in the South American rainforest, the ants typically harvest around a fifth of the annual growth - more vegetation than any other animal. In a single lifetime, a leaf-cutter colony turns over and aerates 40 tonnes of soil.

The farms can include livestock, too. Many ant colonies keep aphids, tranquillising them with drugs to keep them docile and "milking" them with their antennae for a sugary honeydew to use as food. Ants also disperse seeds, particularly in the desert regions of Africa and Australia, and several species of plants are so reliant on the insects that they have evolved special structures on the outsides of their seeds, called elaiosomes, that ants can use as food.

All day, every day, the world's ant nests are active: scouting, processing food, fighting and tending to the young. This cohesive working is so important for the wellbeing of the ants that any individuals removed from a colony quickly die.

Studying these co-operative societies has revealed many other fascinating details. In 2007, Franks found that certain species of army ants use their bodies to plug potholes in the forest floor, thus creating a flatter surface for columns of foraging ants from their colony to run back to their nest. Different ants fill holes of different sizes and two or more ants band together if there is a particularly big hole.

This complex group behaviour is organised using chemicals called pheremones. Around two dozen different chemicals guide the instincts of each insect, telling them which way to turn out of the nest to find food, which of their nest mates is dying and needs to be removed from the colony, which ants need feeding, which are the soldiers, which is the queen and which ones process the colony's rubbish. Rub these chemicals out and the ants get lost and colonies fall apart. Put a pheremone in the wrong place, or remove it from a trail to food, and the ants get confused.

The chemicals are also at the root of how a colony makes sophisticated group decisions. Take the example of finding a site for a new nest. When a scout discovers a potential site, she will measure attributes such as floor area, light levels and the size of the entrances. If it is a suitable site, she will return to the colony and teach fellow scouts the way to the new site, using a process called tandem running. "In this way, a number of ants can build up a new nest site and if that number exceeds a threshold, which we call the quorum threshold, they'll switch from tandem running, which is excruciatingly slow, to picking up their nest mates and carrying them," says Franks.

Ants decide what to do individually and also on the basis of pheremones, but when the number of ants that have made a particular decision reaches tipping point (which is flexible), the entire colony is then committed to the decision. "They have thresholds for doing things - one for leaving the old nest if it's not particularly good, one for accepting a new nest if they encounter it and begin tandem running. They then have a threshold for switching from tandem running to carrying if they encounter a quorum in the new nest," says Franks.

What makes ants far more than a scientific curiosity is that this extraordinary collective behaviour from what are, at heart, chemical-sensing automatons, hints at lessons for similar systems in humans too. Neurons are individually relatively dumb but, with billions of them working together in our brains reacting to levels of neurotransmitter chemicals, something creative and remarkable emerges. "Maybe our own brains are using these thresholds," says Franks. "When you model ants and when you model the brain, there are some great similarities. When our brains are deciding, from visual input, whether to move our eyes to the right or the left, populations of neurons and thresholds are obviously involved."

His colleague at Bristol, computer scientist James Marshall, recently used computer models to show that groups of neurons in the primate brain seem to make decisions in roughly the same way as an ant colony. The results, published recently in the Journal of the Royal Society Interface, drew the first formal parallels between decision-making brain circuits in the primate brain and colonies of social insects.

The parallels are not exact, however: ants are more amazing still. As well as using chemicals, ants also communicate by clicking and singing to each other, creating sound by rubbing together their body parts. Jeremy Thomas of the University of Oxford has just shown that these communications are often used against ants by other species: some ants will take care of impostors in their nest - caterpillars, say - if the impostors simply make the right sounds. By playing the sounds of a queen into a colony, Thomas showed that worker ants stood to attention and even greeted the miniature speakers from which the sounds came out. Scientists knew that ants made sounds when in distress, but Thomas's work, published in the journal Science, showed the sounds were more sophisticated and could convey a wider range of messages than previously thought.

For scientists such as Franks, ants are an ideal model species to test out ideas of evolution. "Ants are the Meccano or Lego of biology - you can take them apart and put them back together. The colony is built out of organisms and, without any ethical qualms, you can vivisect a colony and rebuild it without stressing the ants or hurting them in any way. They're an exquisite model system for asking questions about how biological systems are organised."

Bert Hoelldobler, a biologist at Arizona State University and long-time research collaborator of Wilson's, says ant colonies provide scientists with an invaluable way to gain empirical data around how living in societies developed. Compare the biological blueprint of an ant society with that of humans, he says, and you quickly see that much of human society is built on culture rather than genetics. The basic blueprints for society in our genes are much simpler than those coded within the ant's.

It is the sophisticated genetic blueprint hidden within ants that led Wilson and Hoelldobler to propose a new class of life: the superorganism. "A superorganism is a closely knit group that divides labour among its members altruistically," says Wilson. "There are individuals who reproduce in the group and are promoted to be reproducers, and those that do not reproduce and are workers. This allows the group to function as a giant organism."

All of which is a neat description of an ant colony. Think of a superorganism as a dispersible creature that can stretch out limbs to be in many places at once, going out to forage for food and then withdrawing into the nest after raking up whatever is around. In their recent book, Superorganism, Wilson and Hoelldobler describe their idea by comparing each ant in a colony with a cell in, say, the human body, each one specialised for a task and working (to its own probable death) for the good of the organism as a whole.

Superorganisms can outcompete individuals because they can summon a group that can oust anything trying to take over the food source. A solitary insect might be cautious about risking its life in defending a piece of food it has found and is likely to back off - that is not true of ants. "They're willing to fight to the death right on the spot," says Wilson. "And it doesn't affect the colony much at all - they lose a little strength when workers die that way but not much, they just keep on going. The queen can just produce more workers."

Extending the analogy further, Hoelldobler says a superorganism has a sort of intelligence where an ant colony acts as a problem-solving unit (or even a simple brain). "When you look at the incredible nest structures of these leaf-cutter ants - 8 metres down, an area of 50 sq metres - no single ant could do that, or even has the concept of it, but the interaction, the behaviour of millions of individuals that react to particular stimuli that are created by other workers, leads to these fantastic structures. An ant colony is a problem-solving instrument, in a way."

As more details of the lives of ants has poured in during recent years, the concept of the superorganism, and its importance, has gained traction. The next big step in biology, according to Wilson, is to find out how individuals groups of social creatures organise themselves into superorganisms. "We live in a world of ants," he says, "and it's time we woke up to our little six-legged neighbours."

Fast jaws and fantastic noses: A close-up on three ant species

Ponerines are the most diverse families of ants, with more than 1,000 species. They are also the most ancient, providing an insight into how the earliest ants lived before they evolved the highly social societies seen today. Ponerines live in relatively small groups and comprise some of the biggest individual ants. They specialise in hunting just a few types of prey and most of the members of a colony can reproduce, leading to a a lot of competition (and therefore low numbers) within a nest.

Trap-jaw ants have the fastest mandibles in the animal kingdom; they can snap shut at speeds of more than 140mph, killing or maiming prey so that it can be brought back to the nest. The mandibles of larger army ants are used in some parts of the world as surgical stitches: ants are placed along a wound and the insects bite into the sides, pulling the skin together. The bodies are cut away, leaving the mandibles in place along the wound.

Desert ants navigate outside the nest using visual landmarks and also smell. Ants normally excrete pheremones to lay trails to interesting objects or to find their way home, but these volatile chemicals would degrade quickly in arid desert conditions. Instead, desert ants, which roam 100 metres or more from their nests, learn the smell of their nest entrance and use it to get home whenever there are too few visual clues.